The present invention relates to a driver circuit for a power element connected to an inductive load. More particularly but not exclusively, the driver circuit is for an IGBT power transistor which incorporates a Zener diode, and the detailed description that follows covering this field of application is for convenience of explanation only.
As is well known, inductive loads, like coils, are usually associated with a suitable power element enabling them to be driven. A driver circuit is then associated with the power element. In case of malfunctions or power blackout, the consequent overvoltage across the primary windings of a coil need to be controlled to prevent coil damages.
A prior approach to meeting this requirement is illustrated in FIG. 1. This figure shows a driver circuit 20 for a power element 2. The driver circuit 20 comprises basically a trigger block 3, a voltage regulator block 4, and a buffer stage 5. The circuit 20 has an input terminal IN coupled to the trigger block 3 to receive a triggering signal TRIGGER. The circuit also has an output terminal OUT connected to a control terminal B of the power element 2. The output terminal OUT is connected to the output of the trigger block 3, which includes a gate resistor Rg.
The buffer stage 5 of the circuit 20 is supplied a first supply voltage reference BATTERY, and includes an operational amplifier 6 whose output OUT1 is fed back to its inverting (xe2x88x92) input. The output OUT1 is coupled to the control terminal B of the power element 2 through a switch SW3.
The amplifier 6 is connected with its non-inverting (+) input to a circuit node IN4. This node is coupled to the first supply voltage reference BATTERY through a switch SW2 and a resistor R1 connected in series, and coupled to a second supply voltage reference GND through a capacitor C. This capacitor C is a part of the regulator block 4, and is connected in a network that further includes a generator G for generating a current Id, and a switch SW1 for connecting the generator G in parallel with the capacitor C.
Plotted against time in FIG. 2 are typical waveforms of current and voltage signals of the driver circuit of FIG. 1, according to the prior art. In particular, the waveforms of trigger signals, a voltage Vcap measured across the capacitor C, and a current Icoll flowing through the power element 2 are shown.
It will be seen, in particular, that the patterns of the voltage and current signals from a time Tx onwards indicate a malfunction situation. At the time Tx, switch SW2 opens at the same time as switches SW1 and SW3 are closed. A driving voltage is applied to the control terminal B of the power element which decreases linearly in time to zero. Accordingly, the power element will be forced to operate with a decreasing output current to zero.
Although advantageous on several counts, this prior approach still has a significant drawback in that the time taken by the current that is flowing through the power element to become null cannot be controlled with any degree of accuracy, because its pattern is dependent on the characteristics of the power elements.
The underlying technical problem of this invention is to provide a driver circuit adapted, in case of malfunctions, to discontinue the coil drive for a sufficiently long time to avoid overshooting at the primary winding and a consequential unwanted sparking at the secondary, and this is irrespective of the characteristics of the power element being used, thereby overcoming the limitations and/or drawbacks associated with driver circuits according to the prior art.
The principle on which this invention stands is one of monitoring the variation of the current flowing through the power element with respect to time by direct control of the current that flows through a conduction terminal of the power element.
Based on this principle, the technical problem is solved by a driver circuit as previously indicated being characterized in that it comprises a voltage comparator stage having a first input connected to the circuit node, a second input connected directly to a conduction terminal of the power element, and an output connected to the control terminal of the power element.